PST mod - donor scope advice

[Peter Drew]

The popular Tal 100 has a push on objective cell which is then held in place by three screws. The tube at this position is reinforced to double the thickness, you would lose this if you cut this end, the manufacturers precise alignment could also be compromised. Anyway, the length to be removed would cut into the nice Tal logo and that would be a pity.

[Steep]

I'd like to say thanks to Ken (Merlin66) who sent me his modifying a pst pdf, it's a very comprehensive document and has already proved helpful. I checked the pentaprism alignment in my pst using his instructions and it's now much better than it was. I can't wait now for all the bits to arrive so I can get started!

[Steep]

Well the bits are coming together, the Tal 100RS arrived today (it's huge!!) and the filter should be here tomorrow. I thought the tube would be plastic but it's not it's aluminium, looks too nice to go cutting it up too

[JamesF]

the Tal 100RS arrived today (it's huge!!)

Not much chance of sneaking one past my wife then?

James

[Steep]

Blind her with flowers

[JamesF]

That would definitely make her suspicious

James

[Peter Drew]

I think you will find that the Tal tube is steel, just a little trickier to cut, needs a fine tooth blade in the hacksaw.

[Steep]

Ah! in that case Tal meet Mr Scorpion saw!

[moriniboy]

As I'm modding a TAL 100R I do not have a 2" focuser to use.

Here is my proposed design to overcome this and allow a bit of adjustment to get the Etalon distance correct.

Can anyone see any problems with this?

[Peter Drew]

Should be fine as far as I can see, once the etalon position has been set there should be no further adjustment necessary.

[Stu]

That's exactly what I am planning. Peter, is it likely to be able to avoid cutting anything the tube if using this method? I'm using a Vixen 80m

Thanks

Stu

[Steep]

Re ERF tilt

Am I right in thinking that the tilt in the 100rs plus 75mm filter should be (10/1000x75) = 0.75mm and if so can it be 1mm or more as long as it's not less?

Also is there an optimal place in the tube for the filter to go? I understand that the halfway point wouldn't be best but should the filter holder inside the tube be nearer the pst for a tighter fit to the tube wall?

/edit not meaning to butt in on Stu's question.

[Steep]

First problem, the Thorlabs holder filter retaining ring was misthreaded on my example and extremely tight to turn, even in a vice and using two screwdrivers it would only just move. By pure luck I solved this, I was trying to see how bad the problem was, tugging the ring outwards with my fingertips when it gave a pop and freed up.

[lukebl]

Hi all,

An update. I've just acquired a TAL mount to go with the TAL 100RS, and I must say what a lovely piece of solid and understated engineering it is. AND it only cost £85 new from A10 Optical (a.k.a Scopes n' Skies, Pulsar Optical, etc). So for less than the price of a tinny and flimsy EQ1 mount, I got something of real quality and weight. Whatever other people's experiences of that particular supplier, it arrived by next-day delivery (or it would have done if I'd been in when the courier first called!). They've still got a few more in stock at that price too.

Very pleased that the gold tube unscrewed with no effort at all. Now waiting for my D-ERF filter to arrive, and then I'll get out the hacksaw to finish the mod.

[JamesF]

Looks very nice, Luke

James

[JamesF]

Once the stage 1 mod has been done, does the donor scope retain it's effective focal length? I'm mostly convinced it does, but wise to check

James

[allcart]

Yes I'm sure it does. I think the only way to change the effective focal length is by changing the lens system of the pst etalon, or incorporate barlows into the optical path. As usual, (the usual disclaimer) I could be wrong. I am only a novice after all.

[allcart]

As a footnote, I chopped 50mm off my C6-R and the only thing that has changed is that the drawtube extends more for normal WL observing. It was to be expected.

[keithatrochdale]

Hi all,

An update. I've just acquired a TAL mount to go with the TAL 100RS, and I must say what a lovely piece of solid and understated engineering it is. AND it only cost £85 new from A10 Optical (a.k.a Scopes n' Skies, Pulsar Optical, etc). So for less than the price of a tinny and flimsy EQ1 mount, I got something of real quality and weight. Whatever other people's experiences of that particular supplier, it arrived by next-day delivery (or it would have done if I'd been in when the courier first called!). They've still got a few more in stock at that price too.

Very pleased that the gold tube unscrewed with no effort at all. Now waiting for my D-ERF filter to arrive, and then I'll get out the hacksaw to finish the mod.

Don't forget to put some Baader film on the sighting scope!!!!!

I cut a circle of film, remove the plastic dewsheild, place the round circle of Baader film over the end of the sighting scope and gently push the plastic dewshield back onto the scope. Trim off any excess with scissors, then push the dewshield on a little more and all the edges of the film should have disapeared.

[Merlin66]

Yes, the final focal length remains unchanged, but the final focal ratio will become f10 ie a 150/600 donor would become an effective 60mm f10.

[allcart]

Hmm! this bit always confuses me.

So my C6-R, 150mm, 1200FL, F8 will become what?

And am I correct in thinking that if I stop it down to 120mm it will then have an effective focal ratio of F10?

[Merlin66]

The lenses and etalon act as aperture stops...so the final effective outcome is always an f10 beam.

So yes, it effectively stops you down to 120mm aperture.

[Peter Drew]

Keep plugging this fact Ken and it will eventually become common knowledge!. :smiley:

[Roel]

3. The length of the gold tube inside the donor may interfer with the baffles and cause vignetting

I don't understand exactly why this should be the case. The light cone of any f/10 donor scope will have the same shape as that of the original 40mm PST objective. If the gold tube can't vignet the PST obvjective, it can't vignet the f/10 donorscope's objective.

(it will certainly vignet light cones faster than f/10, but the etalon assembly itself would do this even with a short nosepiece)

[Roel]

When you unscrew the eyepiece/ BF5 section from the "black box" you'll find that some of the Celestron 1.25" diagonals have exactly the same thread on their eyepiece section (!!?)

That's right. I used an old "Celestron Star Diagonal" to screw the PST eyepiece filters on. It must be said that the diagonal houses a prism, not a mirror. This is important because the threaded part on the PST filter is a a bit longer than that of the original Celestron eyepiece holder. When screwed in, the PST filter will hit the glass of the prism, causing astigmatism or misaligment of the prism! Therefore somekind of spacing ring is necessary: I used a simple rubber O-ring: